Data communication systems and methods are used in the transmission of information for an increasing variety of purposes, including the control of equipment. As such, improving the performance of data communication systems has become an important focus of attention. For example, optical communication systems are continually undergoing improvement in many areas related to transmission performance such as capacity, bandwidth, and instantaneous data transmission rate.
Certain communication networks require that signals be transmitted continuously, in order to ensure that clock and data recovery devices (e.g., including phase locked loop (PLL) devices) at the receiver are always synchronized and locked to receive the transmitted data. In such networks, if no payload data is awaiting transmission, a special “idle” signal is transmitted. The idle signal maintains the clock and data recovery devices in a synchronized and locked state.
Other data communication systems and methods involve the use of signals that include “burst mode” data. In burst mode data communication, one or more data packets are transmitted substantially continuously over a signal channel during a data transmission time interval. During another quiescent time interval, the signal channel is substantially free of signals. Accordingly, in some data transmission schemes a plurality of quiescent time intervals are disposed chronologically between a corresponding plurality of data transmission time intervals. The combination of the data transmission time intervals and the quiescent time intervals is known as a “datastream.” The quiescent time intervals are referred to as “gaps” in the datastream.
Burst mode data transmission is employed in various applications including automatic control applications. For example, burst mode data transmission is included in various “fly-by-wire” vehicle control systems for vehicles such as wheeled vehicles and aircraft.
An exemplary fly-by-wire control system is used to actuate the aerodynamic control surfaces of an airplane. In such a system, a transducer in an airplane cockpit detects a motion of, for example, a steering yoke. The transducer produces a control signal such as an optical control signal. The signal is conveyed to an actuator over a communication medium. In the case of an optical control signal, a communication medium such as an optical fiber is used to couple the control signal from the transducer to the actuator. The control signal is received at the actuator and the actuator responsively applies a force through a mechanical linkage to an aerodynamic surface of the airplane. For example, the actuator causes a force that pivots an elevator surface in the tail of the airplane.
In some control systems, including some fly-by-wire systems, local servo devices maintain a substantially constant orientation of the control surface until a change is ordered by an action of the transducer. In such a system, active control signals are transmitted between the transducer and the actuator primarily when a change in control surface position is required. The result is a control signal that includes intervals of active data transmission and quiescent intervals. As noted above, such data transmission is referred to as burst mode data transmission.
Burst mode data transmission is also employed in other communication systems, such as computer network and telephony systems. In such systems, it is advantageous to maintain a quiescent communication channel when no payload data is available for transmission. For example, in an optical communication network, a light source, such as a laser, may be used for signaling between two devices. When no data is available for transmission, it may be preferable to extinguish the light source. By turning off the light source during quiescent periods, an operational lifetime of the light source may be extended, power may be conserved, and a risk of personal-injury to, for example, maintenance personnel, may be reduced.
FIG. 1. shows a time domain graph of an exemplary burst mode digital data signal 100. The digital data signal 100 includes bursts 102, 104 of one or more packets of data separated by gaps 101, 103, 105 during which no data is transmitted. In some systems where data is not transmitted continuously, clock and data recovery devices are required to re-synchronize and re-lock for each individual burst or packet of data. To enable such re-synchronization and re-locking, a “preamble” is attached to each burst or packet. The preamble follows a pattern predefined by a communication protocol. An exemplary protocol preamble includes a succession of alternating high and low values (i.e., “1” and “0”) prepended to the data as illustrated in FIG. 2.
FIG. 2 shows a time domain graph of an exemplary datastream 200. The datastream 200 includes a data burst 202 between gaps 201, 203 of the datastream. In some circumstances, it is desirable to detect the arrival of a data burst after a data gap. For example, receiving equipment or automatic gain control devices may be activated and deactivated depending on whether data is present. Accordingly, it is useful to have a signal presence detector that detects the arrival of a burst of data after a data gap.
It is known to use an integrating device to detect the arrival of a data signal. For example, a diode may be placed in series with the arriving signal. A current flowing through the diode is allowed to accumulate on a capacitor. When an electrical potential across the capacitor reaches a threshold voltage, a signal is deemed to be present. A disadvantage of this arrangement is that it does not respond rapidly to an arriving signal. The time constant of such a system can be shortened by various methods such as using a differential input signal and a corresponding pair of diodes. Because time is required for the capacitor(s) to accumulate electrical charge prior to indicating single arrival, it is difficult to respond to a first transition of an incoming signal.